1. Field of the Invention
The present invention relates to an ink jet recording head that uses heat energy to eject ink droplets toward a recording medium.
2. Description of the Related Art
Japanese Patent Application Laid-Open Publication (Kokai) Nos. SHO-48-9622 and SHO-54-51837 disclose ink jet recording devices with a print head that uses a pulse of heat to rapidly vaporize a portion of ink filling an ink channel. The expansion of the vaporized ink ejects an ink droplet from an orifice in the print head. The simplest method for developing the pulse of heat is to energize a thin-film resistor with a pulse of voltage. Specific configurations for print heads including thin-film resistors are disclosed in the Dec. 12, 1988 edition of Nikkei Mechanical (page 58) and in the August 1988 edition of Hewlett-Packard Journal.
A conventional heater includes a thin-film resistor and conductor. Because the heater is used in a water-based electrolyte ink, protective layers are required for protecting the thin-film resistor and conductor against oxidation, corrosion, galvanization, and cavitation. For this reason, the thin-film resistor and conductor are covered with an approximately 3.0 .mu.m thick anti-oxidation protective layer. The anti-oxidation protection layer is further covered with an approximately 0.5 .mu.m thick anti-activation protective layer. The anti-activation protective layer is formed from a Ta metal thin-film.
An energy of 15-30 .mu.Joule/pulse is required to heat ink through the two protective layers. However, most of this energy is consumed as heat that escapes through the substrate of the head. Also, the volume of ejected droplets is liable to be varied, resulting in fluctuation in density of printed images.
Japanese Patent Application Laid-Open Publication (Kokai) No. HEI-08-238771 discloses a method for forming a protection-layerless Ta--Si--O tertiary alloy thin-film resistor. An oxidation layer having a thickness of 10 to 20 nm is formed on the surface of the thin-film resistor using thermal oxidation. The oxidation layer has superior electric insulation properties and mechanical strength and so prevents oxidation, corrosion, and galvanization of the thin-film resistor. Further, energy required to eject a droplet is reduced to about 2.5 .mu.Joule/pulse.
In order to make maximum advantage of the properties of the Ta--Si--O tertiary alloy thin-film resistor, the thin-film conductor used with the Ta--Si--O tertiary alloy thin-film resistor must also be made from a material that is not corroded by ink. The present inventor discovered that nickel or nickel subjected to metal plating is the optimum material for the thin-film conductor.
Also, the present inventor proposed methods for preventing galvanization of the thin-film conductor, in which the thin-film conductor disposed adjacent to an individual electrode is covered with a thermal resistant resin partition.
Japanese Patent Application Laid-Open Publication (Kokai) No. HEI-8-238771 and Japan Hard Copy '96 (page 169), which was published in July 1996 for the Annual Conference of Japan Hardcopy for the Society of Electrophotography of Japan, disclose a head structure that can protect the thin-film resistor from cavitation damage. Protecting the thin-film resistor from cavitation damage can increase the life of the head.
When the thin-film resistor and its driver circuit are formed on the same silicon wafer, then thermal oxidation of the Ta--Si--O tertiary alloy thin-film resistor must be performed at a low temperature. However, when the temperature used during thermal oxidation of the Ta--Si--O tertiary alloy thin-film resistor is low, the resultant oxidation layer can be thinner than 10 nm. Such a thin-film has insufficient strength to protect the thin-film resistor to reliably guarantee a head life of 100 million pulses or more.